344 
cylinder asc, Fig. 3, and having their centres of gravity coin- 
Fie. 3. 
- ciding. The pressure of the water on pc balances the weight 
of either of them, considered separately, but the pressure of 
the water on rc tends to make the parallelopiped rotate in the 
same direction as that on pc, which tendency not being in the 
same ratio for the angular motion as that exerted on pc, the 
two figures cannot act in the same manner, nor can any other 
figure, where the action on the periphery produces a rotating 
effect, except on the one given part pc. Buta part, or parts, 
might be excentric in one direction, provided they be coun- 
teracted by others in an opposite direction. 
The semi-cylindrical float described above is capable of 
being applied to many useful purposes; the delicacy of its 
action, when properly balanced, is such, that a solid body ca- 
pable of raising the surface level in the cistern, in which the 
compensator may be placed, only 1-2000th ofan inch, it will, on 
being gently plunged into the water, cause an elevation of the 
float quite visible, and indicating a movement, perhaps, equal 
to the bulk of the immersed solid. It is, therefore, applicable 
to the measurement of complicated structures, such as groups 
of crystals, or masses of other matter, and by it also specific 
gravities, expansion of solids, &c., may be ascertained with 
